This invention relates to integrated circuits, and more particularly, to oscillator circuitry for integrated circuits.
Digital integrated circuits often contain analog circuitry. For example, some digital integrated circuits contain analog phase-locked loop circuitry.
Analog circuits such as phase-locked loops may contain voltage-controlled oscillators. A typical voltage-controlled oscillator is formed from a number of inverters connected in a loop. Varactors may be provided at the output of each of the inverters. The varactors may serve as controllable sources of capacitive loading. An analog control voltage may be applied to each varactor to control the capacitance produced by that varactor. When the varactors are adjusted so that they produce relatively large capacitances, the inverters in the loop will switch relatively slowly and the output of the voltage-controlled oscillator will have a relatively low frequency. When the varactors are adjusted so that they produce relatively small capacitances, the inverters in the loop will switch relatively quickly and the output of the voltage-controlled oscillator will have a relatively high frequency.
As integrated circuit dimensions shrink with advances in integrated circuit processing technology, it is becoming increasingly difficult to migrate previous generations of analog circuit designs to next generation platforms. This is because analog circuits may be sensitive to changes in transistor speed and other circuit parameters that are directly affected by the alterations that are made to the transistors and other components on an integrated circuit when using an updated fabrication process.
Analog circuits may also be more susceptible to noise than digital circuits. For example, analog circuits such as voltage-controlled oscillators may be susceptible to noise on their voltage control inputs. Digital designs may be more immune to undesired changes resulting from process updates. Digital designs may also be more accurate than analog designs in some circumstances.
It would therefore be desirable to be able to provide digitally controlled oscillator circuits for integrated circuits.